1. Field of the Invention
The present invention generally relates to the field of well drilling operations, and more specifically to methods and systems for positioning downhole tools in a wellbore. In particular, the present invention discloses a method for more accurately adjusting the position of downhole tools by accounting for additional tool movement that occurs within the wellbore as a result of various external factors.
2. Description of the Problem and Related Art
A well is formed by drilling a borehole into the earth to recover fluids, such as hydrocarbons, natural gas, or water. The borehole may be lined with a casing that is held in place by surrounding cement along the borehole to form a wellbore. The well may be classified as a vertical, deviated, or horizontal well. Vertical wells are drilled toward a target that is generally beneath the surface location of the drilling equipment. Deviated wells are wells that are drilled into the earth's surface at an angle. Horizontal wells extend from a vertical or deviated well and are parallel to the earth's surface.
After the borehole has been drilled but before the casing is set, measurements may be taken along the length of the borehole by running sensing and recording instruments through the open hole to produce an open hole log that correlates readings of various subterranean formations with the depth at which the measurements were made. This open hole log generally takes the form of a graph of measurements taken by the particular instrument along one axis and the depth within the well along an orthogonal axis. These measurements, or readings, are typically characteristics of the subsurface formation, such as natural gamma ray radiation, resistivity, and porosity. Based on these readings, a well operator is able to identify and locate zones of interest that may potentially produce desirable fluids, such as hydrocarbons.
If the well operator identifies one or more zones of interest, the open hole is cased to provide a wellbore. Casing generally consists of a string of steel pipes that are connected by casing collars and lowered down the borehole and held in place by a surrounding layer of cement between the casing and the borehole. Casing may also be made of other materials, such as fiberglass or polypropylene, and connected without the use of collars. This wellbore structure prevents the walls of the borehole from collapsing and allows sections of the wellbore to be isolated from other sections.
After the wellbore is complete, a second log is typically run within the wellbore to correlate selected locations along the casing with the zones of interest identified in the open hole log. Casing devices, i.e. markers attached to or incorporated into the casing, may be identified and located in this manner. Casing collars that are used to connect two sections of pipe are frequently used as markers. Other types of casing devices may be used, such as magnetic devices, radioactive devices, and radiofrequency devices.
After the casing has been inserted and cemented within the borehole, there are a number of operations that can be performed in order to complete the well. Different downhole tools may be used to perform these operations, depending upon the desired result. For example, a perforating gun may be used to punch holes in the casing and cement so that fluid can flow from the subsurface formation into the well and be brought to the surface. As another example, bridge plugs, packers, and cement retainers can be used to isolate a zone of interest from other zones within the well. Occasionally a decision may be made to create a borehole laterally from the main wellbore, which requires the insertion of a whipstock at a given depth within the wellbore so that a drill bit is urged to penetrate the casing at that depth. Downhole tools must be placed at the correct location in order to perform their function and achieve the desired result.
Downhole tools are inserted into the wellbore on a high strength steel cable, or wireline, which is controlled by surface equipment. The surface equipment controlling the movement of the wireline generally includes a power source; a hydraulic drive unit (or a transmission on some of the older units); a spool or other storage device around which the wireline is coiled, which is actuated and controlled by the hydraulic drive; a measurement mechanism in contact with the wireline between the spool and the wellhead to measure the length of the deployed wireline; and an odometer that records the length of the wireline fed into the wellbore. One or more downhole tools may be attached to the wireline as a tool string. The tool string generally includes a detector that can accurately locate the casing devices so that downhole tools can be positioned at a selected depth. The detector transmits signals to an indicator, which is generally part of the surface equipment.
Generally, the tool string is lowered to a depth below the depth where the downhole tools are ultimately to be positioned. The tool string is then raised at a rate of speed recommended for the type of detector so that the wireline is in tension. As an alternative, the detector may be used to locate the casing devices as the downhole tools are lowered into the wellbore. The odometer of the surface equipment is then calibrated to correspond to the locations of the casing devices as recorded in the correlation log.
Regardless of whether the downhole tools are raised or lowered to the correct depth, the surface equipment is stopped when the selected casing device within the wellbore is located. When the surface equipment is stopped, however, the downhole tools do not immediately stop moving. The movement of the downhole tools after the surface equipment has stopped is frequently called “creep.” Some of the factors that may influence the amount of creep include wireline speed, the weight of the downhole tools, depth, wireline size, wellbore deviations, the outer diameter of downhole tools relative to the inner diameter of the casing, and the viscosity of fluid in the casing. Unless creep is accurately determined and the position of downhole tools adjusted accordingly, the downhole tools will not be in the correct position to accomplish the desired result.
Failure to account for creep may result in downhole operations being performed at the incorrect depth or being performed multiple times before the correct depth is reached. Perforations or downhole equipment such as bridge plugs, packers, and cement retainers may be set at the incorrect depth, resulting in excessive gas or water production. Even worse, when the zones of interest are relatively thin, failure to account for creep could result in perforations that are above or below the zone, making a potentially productive well appear to be a “dry hole.” The well would then be plugged and abandoned due to lack of production, resulting in the loss of potential revenue from production as well as the money invested to investigate the site, obtain the mineral rights, and drill and complete the well. In addition, the entire subsurface field may be abandoned for lack of production, further compounding the economic loss.
Others have considered methods for placing downhole tools at a selected depth within a wellbore, and for background information relating to such placement, reference may be made to the following United States patents and patent publications:
U.S. Pat. No.TitleInventor3,145,771Well Operation Depth Control MethodPennebaker3,291,207Well Completion MethodRike3,396,786Depth Control Methods and ApparatusSchuster et al.3,396,787Depth Control Methods and ApparatusVann3,396,788Depth Control Methods and ApparatusBell3,497,958Systems and Methods for DeterminingGollwitzerthe Position of a Tool in a Borehole4,327,412Well Logging Data ProcessingTimmonsTechnique5,279,366Method for Wireline Operation DepthScholesControl in Cased Wells6,516,663 B2Downhole Electromagnetic LoggingWonginto Place Tool6,736,210 B2Apparatus and Methods for PlacingHosie et al.Downhole Tools in a Wellbore7,073,582 B2Method and Apparatus for Positioning aConnell et al.Downhole ToolPub. No.TitleInventor2004/0221986 A1Apparatus and Methods for PlacingHosie et al.Downhole Tools in a Wellbore2005/0199392 A1Method and Apparatus forConnell et al.Positioning a Downhole Tool
For example, U.S. Pat. No. 3,396,787, to Vann, discloses a method and apparatus for accurately positioning a well tool in a wellbore by adjusting pre-existing settings obtained from well logs, according to casing collar depths as the casing collars are detected by the well tool during descent or ascent. The disclosure acknowledges that slight errors will occur as the cable is being unreeled, but does not provide a method for adjusting for these errors before the casing collar depths are determined nor does it address the problem of creep after the cable is stopped.
U.S. Pat. No. 3,497,958, to Gollwitzer, discloses a method and system for accounting for cable stretch, which measure the difference in tension between the cable end at the tool and the cable end at the surface of the earth, and then corrects the cable length measurements derived from the sheave wheel device according to the tension difference. The disclosure goes on to correct for sheave wheel calibration errors and temperature effects on the cable stretch, but does not take into account other factors, e.g. viscosity of the fluid through which the downhole tool is immersed, angle of the wellbore, etc., nor does it address the problem of creep after the cable is stopped.
U.S. Pat. No. 4,327,412, to Timmons, discloses a method of correlating the results of two or more well logs in order to compensate for errors in measurements. The disclosure acknowledges that cable length measurement devices, i.e. sheave-wheel devices, do not accurately take errors caused by cable stretch into account. It goes on to describe a statistical method to compensate for such errors by comparing two or more well logs recorded by different recording devices or by successive runs of the same recording device in order to account for erroneous depth displacements. However, the method does not provide a method, once the corrective displacement is known, of accurately positioning a tool or recording device within a wellbore to account for creep nor does it address the problem of creep after the cable is stopped.
U.S. Pat. No. 6,736,210, to Hosie et al., discloses the use of a collar detector to communicate a depth position of an apparatus in a wellbore to the surface of the well. U.S. Pat. Pub. No. 2004/0221986, to Hosie et al., also discloses the use of a collar detector to communicate a depth position of an apparatus in a wellbore to the surface of the well. However, the method does not specifically address the problem of tool movement within the wellbore after the surface equipment is stopped.
None of these references specifically accounts for the problem of creep after the surface equipment has stopped. As a result, use of these methods does not take into consideration the movement of the tool resulting from creep, once the tool has been placed within the wellbore. This incorrect placement may result in operational delays, additional costs, and even the abandonment of potentially productive wells or fields.